I think this is what you are looking for.
field calc it into a new geometry field then field map it to your geometry field of you final layer, Long_Double and Lat_Double are your field names in this example
'{' + ' "x":' + Long_Double + ',' + ' "y":' + Lat_Double + ',' + ' "spatialReference":{"wkid":4326} }'
Calculate Geometry with GeoEvent